Inhibition of protein phosphorylation modulates expression of the Jak family protein tyrosine kinases

Treatment of murine Friend cells with a dose of the protein kinase inhibitor staurosporine, which is able to block the response of the cells to interferons, appears to inhibit phosphorylation of Jak proteins and, interestingly, to specifically reduce tyk2 and Jak1 expression and to increase Jak2 both in the presence and in the absence of interferons. Therefore, a potential role for phosphorylation events in the regulation of expression of the Jak family members is suggested.

Influence of transmembrane domains on the fusogenic abilities of human and murine leukemia retrovirus envelopes

The envelopes of two highly divergent oncoviruses, human T-cell leukemia virus type 1 (HTLV-1) and Friend murine leukemia virus (F-MuLV), have distinct patterns of cellular receptor recognition, fusion, and syncytium formation. To analyze the influence of the transmembrane envelope subunit (TM) on fusogenic properties, we substituted either the entire TM or distinct domains from F-MuLV for the corresponding domains in the HTLV-1 envelope. Parental, chimeric, and truncated envelopes cloned into a eukaryotic expression vector were monitored for fusogenic potential in human, rat, and murine indicator cell lines by using a quantitative assay. This highly sensitive assay allowed us to assess the fusogenic properties and syncytium-forming abilities of the HTLV-1 envelope in murine NIH 3T3 cells. All chimeric envelopes containing extracellular sequences of the F-MuLV TM were blocked in their maturation process. Although deletions of the HTLV-1 cytoplasmic domain, alone and in combination with the membrane-spanning domain, did not prevent envelope cell surface expression, they impaired and suppressed fusogenic properties, respectively. In contrast, envelopes carrying substitutions of membrane-spanning and cytoplasmic domains were highly fusogenic. Our results indicate that these two domains in F-MuLV and HTLV-1 constitute structural entities with similar fusogenic properties. However, in the absence of a cytoplasmic domain, the F-MuLV membrane-spanning domain appeared to confer weaker fusogenic properties than the HTLV-1 membrane-spanning domain.

Erythroleukaemia induction by the Friend spleen focus-forming virus

The Friend spleen focus-forming virus has been a valuable tool for understanding the molecular events involved in the multiple stages of leukaemia. As summarized in Figure 3, the primary effect of SFFV, which occurs within days, is to cause a polyclonal proliferation of erythroid precursor cells that can proliferate in the absence of their normal regulator erythropoietin. This is the direct result of the unique envelope glycoprotein encoded by SFFV, which is transported to the cell surface and apparently interacts with the EpoR or another component of the multimeric EpoR complex, resulting in the constitutive activation of the Epo signal transduction pathway. Within this proliferating population of erythroid cells is a rare cell that has undergone several genetic changes due to the integration of the viral genome in specific sites in the mouse DNA. This leads to the activation of a gene encoding the PU.1 transcription factor, whose high expression in erythroid cells may be the cause of the block in differentiation that is characteristic of SFFV-transformed erythroid cells. SFFV integration can also lead to the inactivation of the p53 tumour supressor gene, giving these cells a growth advantage in the mouse. The disease induced by SFFV in mice is very similar to polycythaemia vera in humans (Golde et al, 1981). The major clinical feature of polycythaemia vera is the continuous expansion of the number of mature red blood cells in the presence of low serum Epo levels. Also, BFU-E and CFU-E from these patients can form in the absence of Epo like the analogous cells from SFFV-infected mice (Casadevall et al, 1982). It is possible that haematopoietic cells from individuals suffering from this disease express a protein similar to the envelope glycoprotein of SFFV that can interact with the EpoR and lead to its constitutive activation. Alternatively, these patients may contain a mutant EpoR gene that is constitutively activated like the mutant EpoR described earlier. As we understand more fully how the SFFV envelope protein constitutively activates te EpoR complex, we can begin to design therapies to counteract its action that can then be applied to treating patients with polycythaemia vera or other human diseases associated with uncontrolled erythropoiesis.

Prevention of retrovirus-induced neurological disease by infection with a nonneuropathogenic retrovirus

Perinatal infection of susceptible mice with the neurotropic retrovirus CasBrE leads to a noninflammatory spongiform degeneration of the central nervous system with a long incubation period of up to 1 year. Virus replication in infected animals can be suppressed by administration of antiviral antibodies, cytotoxic T cells, or by AZT treatment, which results in partial to complete protection from neurological disease. A highly neuropathogenic chimeric retrovirus, FrCasE, which contains the envelope gene of CasBrE, induces rapid neurodegeneration within only 16 days. Here we report that this fatal disease could be prevented if a nonneuropathogenic Friend murine leukemia virus was administered to mice prior to their infection with FrCasE. This double inoculation led to a substantial reduction of the replication level of FrCasE in spleen and CNS. Only live but not heat-inactivated nonneuropathogenic virus was able to protect from FrCasE-induced neurological disease. The extent of protection was influenced by the viral envelope gene and the kinetics of replication of the nonneuropathogenic virus. These observations in addition to the rapidity of the effect make it likely that competition for replication sites through the mechanism of viral interference is responsible for the protection. Resistance was demonstrable in vivo even when the "protecting" and "challenge" virus belonged to different in vitro interference groups. However, the protection was considerably weaker than that seen between viruses belonging to the same interference group.

Erythropoietin cell biology

Erythropoietin (EPO) mediates its biologic actions in responsive cells through the 78 kDa cell surface form of the EPO receptor. The biosynthesis, activations, and destruction of EPO receptors are described. The binding of EPO to its receptor leads to the activation of a tyrosine protein kinase(s) that results in the phosphorylation of the receptor and other proteins.

Inhibition of Friend murine leukemia virus activity by guanosine/thymidine oligonucleotides

Oligonucleotides consisting of only deoxyguanosine and deoxythymidine were stable in culture and were able to significantly inhibit Friend Murine Leukemia Virus (FMLV) production in acute cell culture assay systems. The oligonucleotides did not share homology with, or possess any complementary (antisense) sequence motifs to the FMLV genome. The guanosine/thymidine-containing oligonucleotides (GTOs) which demonstrated anti-FMLV activity in acute infection assays were synthesized with natural phosphodiester (PD) linkages (backbones). The observed antiviral activities of these oligonucleotides increased significantly when the PD backbone was replaced with a phosphorothioate (PT) backbone. Experiments designed to investigate a potential antiviral mechanism of action demonstrated that oligonucleotides tested were capable of blocking virus adsorption. In addition, GTOs with PD backbones were competitive inhibitors of FMLV reverse transcriptase (RT). When the same experiments were performed using oligonucleotides with PT backbones, all compounds tested demonstrated significant competitive inhibition of FMLV RT. The measured inhibitory activity of all compounds tested in culture assays was enhanced by at least a factor of 10 when the PD linkages were replaced with PT. The enhanced antiviral activity exhibited by the sulfur group on the oligonucleotide backbone, and the lack of any designed, sequence-specific interactions, suggest that a large percentage of the reported antiviral activity of oligonucleotides containing a phosphorothioate backbone is due to factors other than rationally designed, sequence-specific interactions. The ability of GTOs to inhibit FMLV in culture, potentially via a number of different mechanisms, makes this a class of compounds which warrants investigation as therapeutic agents to be used against retroviral infections.

A nonstructural gag-encoded glycoprotein precursor is necessary for efficient spreading and pathogenesis of murine leukemia viruses

In addition to the Gag-Pol and Env precursors whose translation initiates at AUG codons, murine, feline, and simian type C oncoviruses also express glycosylated Gag-Pol precursors (glycoGag), glycoGag translation is initiated at CUG codons located upstream of the Gag AUG initiation codon. In contrast to Gag, glycoGag is translocated into the endoplasmic reticulum and is absent from virions. Since glycoGag has been described to be dispensable ex vivo, we investigated the in vivo effects of a glycoGag- mutation in the Friend murine leukemia virus (F-MuLV). F-MuLV induces severe early hemolytic anemia and subsequent erythroleukemia within 2 months after inoculation of newborn mice. We obtained a glycoGag- F-MuLV, strain H5, by inserting an octanucleotide linker downstream of the CUG codon leading to the reading of a stop codon in all reading frames upstream of the Gag AUG. F-MuLV H5 did not induce severe early hemolytic anemia, and latency of erythroleukemia was significantly increased most likely because of an approximately 1-week delay in the in vivo spreading. Accordingly, induction of recombinant polytropic viruses was also significantly delayed. Close examination of ex vivo spreading kinetics also showed a slower dissemination of F-MuLV H5. Western blot (immunoblot) performed after inoculation of newborn mice with this glycoGag- virus indicated the emergence of new glycoGag+ viruses. PCR analyses with F-MuLV-specific primers demonstrated in vivo pseudoreversions restoring the glycoGag reading frame. Our results demonstrated that glycoGag expression is positively selected and essential for full spreading and pathogenic abilities.

Accumulation of unintegrated viral DNA in rat glial cells infected with neurotropic Friend murine leukemia virus

FrC6 virus isolated from Friend murine leukemia virus (FLV) as a neurotropic virus clone induced a high-degree of accumulation of unintegrated viral DNA during infection of rat glial cells by 72 h post-infection. When anti-FLV neutralizing antibody, dextran sulfate, or 3'-azido-3'-deoxythymidine (AZT) was added to the culture within 24 h after infection, the accumulation of unintegrated viral DNA was inhibited. However, after 30-36 h post-infection, addition of anti-FLV antibody or dextran sulfate scarcely inhibited the accumulation of the unintegrated viral DNA, while addition of AZT at 30-36 h post-infection still reduced the amount of unintegrated viral DNA. Our results demonstrate that after 30-36 h post-infection, when second-round infection had already taken place, further superinfection with cell-free virus to glial cells was not required for the accumulation of unintegrated viral DNA. Possible mechanisms are discussed.

Intracerebral hemorrhages and syncytium formation induced by endothelial cell infection with a murine leukemia virus

The mechanisms of endothelial cell damage that lead to cerebral hemorrhage are not completely understood. In this study, a cloned murine retrovirus, TR1.3, that uniformly induced stroke in neonatal BALB/c mice is described. Restriction digest mapping suggests that TR1.3 is part of the Friend murine leukemia virus (FMuLV) family. However, unlike mice exposed to other FMuLVs, mice infected with TR1.3 virus developed tremors and seizures within 8 to 18 days postinoculation. This was uniformly followed by paralysis and death within 1 to 2 days. Postmortem examination of TR1.3-inoculated mice revealed edematous brain tissue with large areas of intracerebral hemorrhage. Histologic analysis revealed prominent small vessel pathology including syncytium formation of endothelial cells. Immunohistochemical analysis of frozen brain sections using double fluorescence staining demonstrated that TR1.3 virus specifically infected small vessel endothelial cells. Although infection of vessel endothelial cells was detected in several organs, only brain endothelial cells displayed viral infection associated with hemorrhage. The primary determinant of TR1.3-induced neuropathogenicity was found to reside within a 3.0-kb fragment containing the 3' end of the pol gene, the env gene, and the U3 region of the long terminal repeat. The restricted tropism and acute pathogenicity of this cloned murine retrovirus provide a model for studying virus-induced stroke and for elucidating the mechanisms involved in syncytium formation by retroviruses in vivo.

Interference established in mice by infection with Friend murine leukemia virus

Retroviral interference is manifested in chronically infected cells as a decrease in susceptibility to superinfection by virions using the same cellular receptor. The pattern of interference reflects the cellular receptor specificity of the chronically infecting retrovirus and is mediated by the viral envelope glycoprotein, which is postulated to bind competitively all cellular receptors available for viral attachment. We established retroviral interference in mice by infecting them with Friend murine leukemia virus and them measured susceptibility to superinfection by challenging the mice with the erythroproliferative spleen focus-forming virus. Infection of approximately 10% of nucleated splenocytes rendered mice 1% as susceptible to superinfection as untreated controls. The magnitude of this effect was the same in mice incapable of producing neutralizing antibodies or genetically deficient for T cells. The results indicated that retroviral interference in vivo was established rapidly with infection of a fraction of the host cell population and that the decrease in susceptibility to superinfection occurred without a detectable contribution by immunologic factors.

Inhibition of murine leukemia viruses by nuclease-resistant alpha-oligonucleotides

We studied the antiviral activity of nuclease-resistant alpha-anomeric oligonucleotides. An alpha-oligonucleotide (20-mer) targeted to the primer binding site (PBS) of murine retroviruses inhibited viral spreading. The inhibition only occurred when the cells had been electropermeabilized in the presence of the oligonucleotide. The PBS sequence is involved in reverse transcription and in translation. The data suggest that the oligonucleotide could perturb reverse transcription activity. Thus, either the oligonucleotide induced a decrease in initiation or it inhibited the extension of the minus or plus strands DNA during reverse transcription. These results show that reverse transcription may be an interesting target for antisense oligonucleotides.

Altered macrophage antigen-presenting cell function following Friend leukemia virus infection

To investigate the mechanism by which Friend leukemia virus (FV) causes immunosuppression, the ability of peritoneal macrophages to mediate antigen-specific T-cell activation following FV infection was examined. Decreased IL-2 production was observed when antigen-primed T cells were cultured with antigen-pulsed macrophages from mice infected with FV, compared to T cells cultured with macrophages from control mice. Macrophages from FV-infected mice demonstrated decreased phagocytic and pinocytic activity, suggesting that antigen uptake may be impaired in these cells. In addition, FV-infected mice had decreased numbers of MHC class II positive macrophages compared to uninfected controls, as measured by immunofluorescence. The alterations in antigen uptake and class II expression observed in macrophages from FV-infected mice may be the result of infection of these cells by FV, which was demonstrated by in situ hybridization using a FV-specific probe. The ability of FV to infect and modulate the functions of macrophages may account, at least in part, for the immunosuppression observed in FV-infected mice.

Sequential activation of genes for heme pathway enzymes during erythroid differentiation of mouse Friend virus-transformed erythroleukemia cells

Changes in the level of transcripts encoding enzymes of the heme biosynthetic pathway as well as those encoding ubiquitous proteins were examined in murine Friend virus-transformed erythroleukemia cells during erythroid cell differentiation induced by chemicals including dimethyl sulfoxide (DMSO). Early changes following DMSO treatment were marked decreases in mRNAs for three ubiquitous proteins, i.e., a 70 kDa heat shock protein (less than 6 h), heme oxygenase and nonspecific delta-aminolevulinate synthase (ALAS) (less than 12 h). These changes were followed by sequential increases in mRNAs for enzymes in the heme biosynthetic pathway. Namely, mRNAs for the erythroid-specific ALAS, delta-aminolevulinate dehydratase, porphobilinogen deaminase and uroporphyrinogen decarboxylase started to increase at 12, 18, 18-24 and 24 h, respectively. Nuclear runoff studies revealed that these changes are largely transcriptional. Treatments with other inducers of erythroid differentiation, e.g., hexamethylene bisacetamide, n-butyric acid and N'-methylnicotinamide, also showed similar effects on mRNAs as those following DMSO. These findings suggest that both suppression of ubiquitous genes and activation of heme pathway enzyme genes are associated with erythroid differentiation, and the former occurs preceding changes in the latter.

Host humoral and cellular immune mechanisms in the continued suppression of Friend erythroleukemia metastases after interferon alpha/beta treatment in mice

DBA/2 mice were injected intravenously with 2 x 10(6) 3C18 Friend erythroleukemia cells (FLC), a cell line resistant to interferon alpha/beta (IFN-alpha/beta). Although daily administration of mouse IFN-alpha/beta markedly increased the mean survival time, most IFN-treated mice continued to harbor FLC in different organs. To investigate the mechanisms responsible for this persistent suppression of FLC growth in IFN-treated mice, we undertook a series of adoptive transfer experiments with sera and spleen cells. Sera from FLC-injected, IFN-treated mice were very effective in conferring protection on DBA/2 mice even when injected systemically (intravenously) 18-24 h before intravenous challenge with FLC. These sera also exhibited antitumor activity when injected subcutaneously or intraperitoneally together with FLC. The protective factor in serum was shown to be an immunoglobulin. FLC-injected, IFN-treated mice developed antibodies to FLC demonstrable by radioimmunoassay and complement-dependent cytotoxicity. Sera from these mice recognized a specific 65-kD FLC membrane antigen(s) not detectable on membrane extracts from RBL-5 or ESb tumor cells, or on normal spleen cells. FLC-injected, IFN-treated mice also developed a specific cellular response demonstrable by transfer of protection with spleen cells injected intravenously or subcutaneously. Analysis of the responsible spleen cell populations indicated that the effector cells were neither T nor B cells. These results demonstrating the importance of host humoral and cellular immune mechanisms in the persistent suppression of FLC in IFN-treated mice may be relevant to the use of IFN-alpha/beta in patients in whom tumors may regress and tumor cells may then remain latent for extended periods of time.

Mechanism of leukemogenesis induced by mink cell focus-forming murine leukemia viruses

The Friend or Moloney mink cell focus-forming (MCF) virus encodes a recombinant-type envelope glycoprotein, gp70, that is closely related to the membrane glycoprotein, gp55, of Friend spleen focus-forming virus (SFFV). We have shown previously that gp55 has the ability to activate cell growth by binding to the cellular receptor for erythropoietin. Here we show that gp70 encoded by either the Friend or Moloney MCF virus also binds to the erythropoietin receptor and that coexpression of the receptor and gp70 in an interleukin-3 (IL-3)-dependent cell line can activate IL-3-independent growth. Furthermore, when the cDNA for the human IL-2 receptor beta chain, which is related by sequence to the erythropoietin receptor, was introduced into this cell line, it became growth factor independent after infection either with SFFV or with one of the two MCF viruses but not with an ecotropic virus. Based on these observations, we propose a mechanism for the early stage of leukemogenesis induced by the MCF-type murine leukemia viruses.

Suppression of murine leukemia virus-mediated 3Y1 cell fusion by expression of mouse MHC class I

Rat 3Y1 fibroblasts transformed by adenovirus type 12 or its E1A gene formed syncytia by cocultivation with Friend murine leukemia virus (MuLV)-producing cells. On the other hand, parental 3Y1 cells and those derivatives induced by other tumor viruses or chemical carcinogen showed no MuLV-mediated syncytium formation [N. Momozaki et al. (1990) Arch. Virol. 115: 123-126]. The expression of major histocompatibility complex (MHC) class I mRNA and antigens was significantly reduced in these Ad12- and E1A-transformed 3Y1 cells. In contrast, other tumor virus-and chemical carcinogen-transformed 3Y1 cells expressed MHC class I almost in normal levels as did parental 3Y1 cells. Furthermore, Ad12-transformed 3Y1 cells which started to express the transfected exogenous MHC class I gene, H-2Ld, showed no more MuLV-mediated 3Y1 cell fusion. These results indicate that the expression of MHC class I on the cell membrane is closely related to the inhibition of 3Y1 cell fusion by MuLV.

Friend virus replication in normal and immunosuppressed C57BL/6 mice

Young adult C57BL/6 mice are resistant to the replication of Friend virus. We show here that this resistance is not absolute. In 7-week old C57BL/6 mice injected with NB-tropic Friend virus iv, high titers of SFFV could be recovered from the spleen at 8 days after infection but by 21 days, no virus was detectable. A single dose of anti-Thy 1.2 monoclonal antibody iv before FV infection permitted continued replication of SFFV in these animals. This finding strongly supports the hypothesis that SFFV replication in C57BL/6 mice is restricted by a T cell-mediated immune response.

Selective syncytium formation by murine leukemia virus in rat 3Y1 fibroblasts transformed by adenovirus type 12 or its E1A gene

3Y1 rat fibroblasts transformed by adenovirus type 12 or by its E1A gene formed syncytia when cocultivated with the Friend as well as other ecotropic murine leukemia-virus-producing cells. In contrast, the parental 3Y1 cells and those transformed by other tumor viruses or by a chemical carcinogen exhibited no syncytium formation.

Host genes conferring resistance to a central nervous system disease induced by a polytropic recombinant Friend murine retrovirus

Infection of certain strains of mice with the ecotropic Friend murine leukemia virus results in the generation of recombinant polytropic mink cell focus-inducing viruses and the development of erythroleukemia. We isolated a Friend mink cell focus-inducing virus (F-MCF-98D) from a Friend murine leukemia virus-infected BALB/c mouse which caused primarily a neurological disease as well as a low incidence of leukemia in susceptible IRW mice. Through genetic studies with the resistant C57BL/10 strain, we identified two genes which correlated with restricted viral replication and resistance to the development of disease caused by F-MCF-98D. One gene correlated with the expression of an endogenous gp70 linked to the Rmcf gene and might act by viral interference. The mechanism of action of the second gene was less clear, but it appeared to be associated with development of an antiviral antibody response.

Immunosuppression after injection of Friend virus into C57BL/6 mice of different ages

Injection with Friend virus (FV) causes immunosuppression in young and old C57BL/6 mice, i.e. it occurs whether or not the virus replicates very briefly or for a long period. There are only minor age-related differences in the extent of immunosuppression, except that suppression appears to persist somewhat longer in old than in young animals.

Distinct segments within the enhancer region collaborate to specify the type of leukemia induced by nondefective Friend and Moloney viruses

The nondefective Moloney and Friend murine leukemia viruses induce T-cell lymphomas and erythroleukemias, respectively, after being injected into newborn NFS mice. In previous studies, we showed that the distinct disease specificities of the two viruses could be switched by exchanging a small segment, about 200 nucleotides in length, encompassing their enhancer regions. This segment included the direct repeat sequence and an adjacent GC-rich region of about 20 nucleotides defined in studies of Moloney murine sarcoma virus enhancer-promoter function (L. A. Laimins, P. Gruss, R. Pozzatti, and G. Khoury, J. Virol. 49:183-189, 1984). The direct repeats of Friend and Moloney viruses are identical in a central core sequence of 32 nucleotides but have sequence differences on either side of this core as well as in their GC-rich segments. To determine whether disease specificity resides in part or in all of the direct repeat and GC-rich region, we constructed recombinants between Friend and Moloney viruses within this segment and tested them for their disease-inducing phenotypes. We found that disease specificity, in particular the ability of Friend virus sequence to confer erythroleukemogenicity on Moloney virus, is encoded throughout the region in at least three separable segments: the 5' and 3' halves of the direct repeat and the GC-rich segment. When just one of these segments (either both 5' halves of the direct repeat, both 3' halves, or just the GC-rich segment) from Friend virus was substituted into a Moloney virus genome, it conferred only a negligible or low incidence of erythroleukemia (less than or equal to 5% to between 10 and 15%). Any two segments together were considerably more potent (35 to 95% erythroleukemia), with the most effective pair being the two halves of the direct repeat. Individual segments and pairs of segments were considerably more potent determinants when they were matched with a genome of the same origin. Thus, although sequences outside the enhancer region are minor determinants of disease specificity when the enhancer is derived entirely from either Friend or Moloney virus, they can play a significant role when the enhancer is of mixed origin. Some recombinant enhancers conferred a long latent period of disease induction. This was particularly striking when the 5' halves of each copy of the direct repeat sequence were derived from Moloney virus and the 3' halves were derived from Friend virus.(ABSTRACT TRUNCATED AT 400 WORDS)

Production and in vivo biologic actions of recombinant mouse interferon alpha 2

We have expressed a recombinant mouse interferon alpha (r.Mu-IFN alpha 2) in Escherichia coli under the control of a tryptophan promoter using a synthetic adaptor formed by annealing two partially complementary oligonucleotides which introduced an ATG start codon and re-established the complete coding sequence of the mature IFN alpha 2 protein in the expression vector. Levels of up to 10(7) reference units of Mu-IFN alpha 2 per liter of culture were obtained using this construction. This recombinant mouse interferon alpha 2 exhibited antiviral activity in mice infected with EMC virus and antitumor activity in mice inoculated with Friend leukemia cells in a manner similar to that of natural mouse interferon alpha/beta.

Fim-1, Fim-2/c-fms, and Fim-3, three common integration sites of Friend murine leukemia virus in myeloblastic leukemias, map to mouse chromosomes 13, 18, and 3, respectively

Three common proviral integration sites, Fim-1, Fim-2/c-fms, and Fim-3, have been described in mouse myeloid leukemias induced by the Friend murine leukemia virus. The nature and function of Fim-1 and Fim-3 are still unknown since no transcript from these loci has been detected so far. To identify these two loci, we undertook their chromosomal localization using restriction fragment length polymorphism detected between C57BL/6 mice and the wild-derived inbred strain of Mus spretus. Using interspecific backcross analysis, we mapped Fim-1 to mouse chromosome 13 and Fim-3 to mouse chromosome 3. Interestingly, Fim-3 is tightly linked to Evi-1, another common integration site of ecotropic virus involved in another model of mouse myeloid leukemogenesis. Fim-2 spans the 5' end of the c-fms gene, which encodes for the macrophage-colony-stimulating factor receptor. We located the c-fms gene on the D band of chromosome 18 by in situ hybridization.